If you think paying a little more for organic food gets you a more nutritious and safer product, you might want to save your money.

A study led by researchers at Stanford University says that organic products aren’t necessarily more nutritional than conventional varieties, and they’re no less susceptible to contamination from disease-causing microbes like E. coli either.

The findings run counter to the commonly believed wisdom. Organic foods are grown without man-made pesticides or heavy reliance on antibiotics and growth hormones to boost yields; organic farmers also use natural-based fertilizers, like manure, and raise livestock in less-confined spaces — all of which some growers say are key contributors to a healthier and sometimes more nutritious product. Consumers who buy organic have been willing to pay up to twice as much for goods with organic labels.

But the latest results, published in the Annals of Internal Medicine, suggest that buyers may be wasting their money. “We did not find strong evidence that organic foods are more nutritious or healthier than conventional foods,” says Dr. Crystal Smith-Spangler, an instructor in the division of general medical disciplines at Stanford. “And both organic and conventional foods seem to have a similar risk of contamination with bacteria, so consumers shouldn’t assume that one type of food has a lower risk or is safer in terms of food-borne illnesses. Both are equally likely to be contaminated.”

For their new study, Smith-Spangler and her colleagues conducted a review of two categories of research, including 17 studies that compared health outcomes between consumers of organic vs. conventional food products and 223 studies that analyzed the nutritional content of the foods, including key vitamins, minerals and fats.

While the researchers found little difference in nutritional content, they did find that organic produce were 30% less likely to have pesticide residue than conventional fruits and vegetables, which makes sense given that organic farmers depend less on synthetic pest-control methods. Neither organic nor conventional foods showed levels of pesticides high enough to exceed food-safety thresholds. And although both organic and conventional meats were equally likely to be contaminated with bacteria, albeit at very low rates, organic chicken and pork were 33% less likely than conventional livestock to harbor bacteria that were resistant to three or more antibiotics (resistance is an indication of possible overuse of the drugs).

Smith-Spangler’s team separated all foods into two general groups — organic and conventional — so the researchers stress that there may be individual differences in the way specific farmers, either those that use organic methods or those relying on conventional ones, grow their plants or livestock. But overall, there doesn’t seem to be much support for the claim that organic foods are more nutritious.

The researchers did find, however, that organic milk and chicken contained higher levels of omega-3 fatty acids, a healthy fat also found in fish that can reduce the risk of heart disease. Organic produce also contained more total phenols than conventional varieties; phenols include flavonoids that work as antioxidants to fight genetic damage that can lead to cancer and even some neurological disorders like Parkinson’s. But these nutritional differences were small, and the researchers were reluctant to make much of them until further studies confirm the trends.

Based on the few studies included in the analysis that compared health outcomes in people eating organic vs. conventional, there was no evidence that one group was healthier than the other. Some of the studies found that children eating organic produce had lower levels of pesticide residue in their urine than those consuming conventional produce, but the numbers were too small to draw any general conclusions.

Smith-Spangler says the findings should help to educate consumers at the market about what they are buying. Organic foods are produced with fewer pesticides and more natural-growing practices, but that doesn’t always translate into a more nutritious or healthier product.